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From: Peter Zijlstra <peterz@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: paulmck@linux.vnet.ibm.com, Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org, mingo@kernel.org,
	torvalds@linux-foundation.org, mark.rutland@arm.com,
	linux-arch@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	Alexander Kuleshov <kuleshovmail@gmail.com>,
	dvyukov@google.com
Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()
Date: Wed, 11 Oct 2017 18:17:52 +0200	[thread overview]
Message-ID: <20171011161752.x6sd6ue4hmzfnffe@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <7484.1507738025@warthog.procyon.org.uk>

On Wed, Oct 11, 2017 at 05:07:05PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > It does not.  In most cases, the barriered version would be
> > smp_store_release().
> 
> Ummm... Is that good enough?  Is:
> 
> 	WRITE_ONCE(x, 1);
> 	WRITE_ONCE(x, 2);
> 
> equivalent to:
> 
> 	smp_store_release(x, 1);
> 	smp_store_release(x, 2);
> 
> if CONFIG_SMP=n?

Almost; it ends up being:

	barrier();
	WRITE_ONCE(x, 1);
	barrier();
	WRITE_ONCE(x, 2);

> (Consider what happens if an interrupt messes with x).
> 
> If it is good enough, should we be using smp_load_acquire() rather than
> READ_ONCE()?

No, smp_load_acquire() is strictly stronger (and far more expensive on
!Alpha).

Dependent loads do not require barriers (except Alpha, and we want to
kill that special case).

  reply	other threads:[~2017-10-11 16:18 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-10-10  0:19 [PATCH RFC tip/core/rcu 0/15] Remove to-be-unneeded smp_read_barrier_depends() Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 01/15] doc: READ_ONCE() now implies smp_barrier_depends() Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 02/15] mn10300: READ_ONCE() now implies smp_read_barrier_depends() Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 03/15] drivers/net/ethernet/qlogic/qed: Fix __qed_spq_block() ordering Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 04/15] fs/dcache: Use release-acquire for name/length update Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 05/15] percpu: READ_ONCE() now implies smp_read_barrier_depends() Paul E. McKenney
2017-10-10 14:08   ` Tejun Heo
2017-10-10 15:30     ` Paul E. McKenney
2017-10-10 15:49       ` Tejun Heo
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 06/15] rcu: Adjust read-side accessor comments for READ_ONCE() Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 07/15] rtnetlink: Update now-misleading smp_read_barrier_depends() comment Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 08/15] seqlock: Remove now-redundant smp_read_barrier_depends() Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 09/15] uprobes: " Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 10/15] locking: Remove smp_read_barrier_depends() from queued_spin_lock_slowpath() Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 11/15] tracepoint: Remove smp_read_barrier_depends() from comment Paul E. McKenney
2017-10-10  0:31   ` Steven Rostedt
2017-10-10  1:12     ` Mathieu Desnoyers
2017-10-10 15:32       ` Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() Paul E. McKenney
2017-10-10  8:39   ` Peter Zijlstra
2017-10-10  9:36   ` David Howells
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 13/15] mm/ksm: Remove now-redundant smp_read_barrier_depends() Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 14/15] netfilter: " Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  8:43   ` Peter Zijlstra
2017-10-10 15:56     ` Paul E. McKenney
2017-10-10  0:22 ` [PATCH RFC tip/core/rcu 15/15] keyring: " Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  0:22   ` Paul E. McKenney
2017-10-10  9:35 ` [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() David Howells
2017-10-10 15:50   ` Paul E. McKenney
2017-10-10 15:54     ` Peter Zijlstra
2017-10-10 16:05       ` Paul E. McKenney
2017-10-11 12:19   ` David Howells
2017-10-11 12:22     ` Will Deacon
2017-10-11 12:54       ` Paul E. McKenney
2017-10-11 14:18         ` Will Deacon
2017-10-11 14:50           ` Paul E. McKenney
2017-10-11 12:58     ` Paul E. McKenney
2017-10-11 15:17     ` David Howells
2017-10-11 15:59       ` Paul E. McKenney
2017-10-11 16:12         ` Peter Zijlstra
2017-10-11 16:24           ` Peter Zijlstra
2017-10-11 16:47             ` Paul E. McKenney
2017-10-11 16:54               ` Peter Zijlstra
2017-10-11 17:06                 ` Paul E. McKenney
2017-10-11 17:11                   ` Peter Zijlstra
2017-10-11 17:34                     ` Paul E. McKenney
2017-10-11 18:43                       ` Dmitry Vyukov
2017-10-11 18:56                         ` Linus Torvalds
2017-10-11 19:28                           ` Paul E. McKenney
2017-10-11 19:59                           ` Dmitry Vyukov
2017-10-11 17:14                   ` Paul E. McKenney
2017-10-11 17:19             ` Mark Rutland
2017-10-11 16:50           ` Paul E. McKenney
2017-10-11 16:07       ` David Howells
2017-10-11 16:17         ` Peter Zijlstra [this message]
2017-10-11 16:19         ` Paul E. McKenney
2017-10-11 15:28     ` David Howells
2017-10-11 16:02       ` Paul E. McKenney
2017-10-10  9:59 ` David Howells
2017-10-10 15:52   ` Paul E. McKenney
2017-10-11 12:21 ` [PATCH RFC tip/core/rcu 0/15] Remove to-be-unneeded smp_read_barrier_depends() David Howells
2017-10-11 12:56   ` Paul E. McKenney

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